A prime mover of a vehicle such as an internal combustion engine has to be operated at a speed higher than a self-sustaining speed, and even when the vehicle is stopped, the engine is activated for the purpose of generation and air conditioning. For these purposes, according to the conventional art, the engine is disconnected from a powertrain (or a gear train) by a clutch depending on the situation so that the engine is allowed to be activated to generate a power for electric generation even in the stopping vehicle. The engine generates not only a power for propelling the vehicle but also an engine braking force resulting from a pumping loss of the engine. When the vehicle is coasting without being powered by the engine, the engine is disconnected from the powertrain by bringing the clutch into disengagement to reduce power loss resulting from rotating the prime mover passively. Such drive mode in which the vehicle is coasting while disengaging the clutch is called a “neutral coasting (abbreviated as N coasting)”. Under the neutral coasting, an inertia force of the vehicle may be utilized effectively so that fuel efficiency can be improved.
When an accelerator pedal or a brake pedal is depressed under the neutral coasting, the clutch is engaged to transmit engine power to the wheels and to apply the engine braking force to the vehicle. During the neutral coasting, the engine speed is reduced to the self-sustaining speed but rotary members of the powertrain are rotated at a higher speed according to a vehicle speed and a speed ratio. In this situation, if the clutch is brought into engagement, the engine speed will be abruptly raised by the inertia force of the vehicle to the speed according to the vehicle speed and the speed ratio. That is, the engine braking force derived from energy for increasing the engine speed is applied to the vehicle. This means that shocks may be caused by the engine braking force if the clutch is immediately brought into engagement upon depression of the accelerator pedal or the brake pedal.
Japanese Patent Laid-Open No. 2010-281384 describes a coasting control system for reducing such engagement shock of the clutch. According to the teachings of Japanese Patent Laid-Open No. 2010-281384, the control device is configured to adjust a speed ratio of a transmission disposed between a clutch and drive wheels during the neutral coasting, in a manner such that a rotational speed of an output member of the clutch is synchronized to that of an input member of the clutch. That is, the speed ratio of the transmission is adjusted in a manner such that an input speed of the transmission is synchronized to an idling speed of the engine.
Thus, the control device taught by Japanese Patent Laid-Open No. 2010-281384, the rotational speed of the input member of the clutch is synchronized to that of the output member of the clutch so that the clutch is allowed to be brought into engagement without changing rotational speed significantly. According to the teachings of Japanese Patent Laid-Open No. 2010-281384, therefore, shocks resulting from a change in a longitudinal acceleration of the vehicle may be reduced. However, when the clutch is brought into engagement, the engine speed is lowered to the idling speed and hence acceleration response would be degraded. That is, even if the throttle valve supplying fuel to the engine is opened widely to increase the engine speed after engaging the clutch, it would take more time to achieve a desired engine speed due to the inertial mass of the engine, the transmission etc. Consequently, it will take more time to achieve a desired drive torque transmitted to the drive wheels.